1. Field of the Invention
The present disclosure relates to an integrated semiconductor laser element integrated of a plurality of semiconductor lasers.
2. Description of the Related Art
As wavelength variable light sources for dense wavelength division multiplexing (DWDM) optical communications for example, integrated semiconductor laser elements, each integrated of a plurality of semiconductor lasers having laser emission wavelengths different from one another have been disclosed (for example, see Japanese Patent Application Laid-open No. 2005-317695). In an integrated semiconductor laser element of this type, semiconductor lasers to be operated are switched to vary wavelength of laser light output, thereby functioning as a wavelength variable laser. To these semiconductor lasers, an optical coupler and a semiconductor optical amplifier (SOA) are sequentially connected. The laser light from the semiconductor laser to be operated passes through the optical coupler, and is thereafter optically amplified by the SOA and output from an output end of the element.
The integrated semiconductor laser element as described above is incorporated in a laser module with a pig-tail fiber for example, to be used. Such a laser module is used as a signal light source, in combination with an external modulator, for long-distance optical transmission in a DWDM optical communication network system for example.
As a signal light source or a local light source for a digital coherent transmission use at a transmission speed of 40, 100, or 400 Gbps, a wavelength variable laser that is capable of outputting laser light of high intensity and narrow linewidth is necessary. For example, in a general example, a light source used in transmission of a dual polarization quadrature phase shift keying (DP-QPSK) scheme at 100 Gbps requires an optical power intensity from a pig-tail fiber of a laser module to be 40 milliwatts or greater and a spectral linewidth to be 500 kHz or less. In another example, transmission of a dual polarization 16 quadrature amplitude modulation (DP-16QAM) scheme at 400 Gbps requires an optical power intensity from a pig-tail fiber of a laser module to be 40 milliwatts or greater and a spectral linewidth to be 100 kHz or less.
In the above-described integrated semiconductor laser element, in order to achieve narrow linewidth characteristics, generally, narrowing of the linewidth of the semiconductor lasers is performed. As such semiconductor lasers, a distributed feedback laser diode (DFB-LD) of single-mode oscillation and with a high yield rate is suitably used. To narrow linewidth of DFB lasers, values of coupling coefficients κ of diffraction gratings in the laser elements and cavity lengths of the DFB lasers (Ldfb) are increased. However, increasing the values of κ and Ldfb deteriorates side-mode suppression ratios (SMSR), and lowers probabilities of single-mode oscillation. It is thus preferable to keep κLdfb, which is a product of κ and Ldfb, down to about 1.5 or less. That is, preferable narrowing of linewidth of the DFB lasers is achieved by increasing Ldfb while keeping κLdfb to about 1.5. However, increasing the value of Ldfb lowers a current-to-light conversion efficiency. To compensate the lowering of the current-to-light conversion efficiency and obtain a desired optical power intensity from the integrated semiconductor laser element, there is a means to increase a driving current of the DFB lasers (first means), or a means to increase an amplification factor of the semiconductor optical amplifier (second means).
In the first means, the increase of the driving current is known to cause a phenomenon called spatial hole-burning within a laser cavity, resulting in widening of a spectral linewidth.
In the second means, a spectral linewidth of laser light output from a semiconductor laser in the integrated semiconductor laser element may be increased upon amplification by the semiconductor optical amplifier. In this case, the spectral linewidth of the laser light output from the integrated semiconductor laser element broadens than a desired linewidth.
Accordingly, there is a need to provide an integrated semiconductor laser element that is able to output laser light of a desired spectral linewidth and a desired optical intensity.